1. Field of the Invention
The present invention is directed to an image forming apparatus and, more particularly, to an image forming apparatus having a toner level sensor and meter for moving toner from an upper toner supply reservoir to a lower supply reservoir.
2. The Prior Art
Image forming devices including copiers, laser printers, facsimile machines, and the like, include a photoconductive drum (hereinafter photoconductor) having a rigid cylindrical surface that is coated along a defined length of its outer surface. The surface of the photoconductor is charged to a uniform electrical potential and then selectively exposed to light in a pattern corresponding to an original image. Those areas of the photoconductive surface exposed to light are discharged thus forming a latent electrostatic image on the photoconductive surface. A developer material, such as toner, having an electrical charge such that the toner is attracted to the photoconductive surface is used for forming the image. The toner is stored in a reservoir adjacent to the photoconductor and is transferred to the photoconductor by the developer roll. The thickness of the toner layer on the developer roll is controlled by a nip, which is formed between the doctor blade and the developer roll. A recording sheet, such as a blank sheet of paper, is then brought into contact with the discharged photoconductive surface and the toner thereon is transferred to the recording sheet in the form of the latent electrostatic image. The recording sheet is then heated thereby permanently fusing the toner to the sheet.
The toner reservoir is normally located within a cartridge that is removably mounted within the image forming device. Once the toner within a cartridge has been used, the cartridge is removed from the image forming apparatus and replaced with one having a new supply of toner. One of the primary factors affecting the cost per page of printing in an image forming apparatus is the capacity of the toner in the cartridge. A toner reservoir that is too small such that it does not contain an adequate supply of toner requires continual replacement which adds expense due to purchasing new cartridges and becomes frustrating to a user who is repetitively shutting down the image forming apparatus to replace the cartridge. However, if the toner reservoir is too large, the pressure of the toner entering the doctor blade nip is too high, and objectionable vertical streaks are produced on the recording sheet.
Another consideration in the design of the toner reservoir is the desire to produce an image forming device having the smallest possible dimensions. This is a key selling point to consumers who desire the small dimensions because the apparatus are easier to manipulate and move, and occupy a minimal amount of desk space in a workstation where available space if often at a premium. To reduce the dimensions, the toner cartridges are often configured around the other components of the image forming apparatus. One design features a more vertically-aligned reservoir having the toner stored vertically above the doctor blade. This design takes advantage of the available space required for the focal distance required by the laser printheads. Although this increases the capacity of the toner, the design results in excessive toner pressure on the doctor blade nip resulting in poor quality images.
Thus, there remains a need for a large capacity toner reservoir that does not place an excessive amount of pressure on the doctor blade nip and does not necessitate a large image forming device.
The present invention provides for a toner reservoir having adequate toner amounts for creating numerous printed images without placing undesirable pressure on the doctor blade nip resulting in printing errors and defects. The toner reservoir is divided into an upper sump region that contains a majority of the toner and a lower sump region. The lower sump holds enough toner to ensure adequate toner is supplied to the photoconductor resulting in good print quality. As the toner within the lower sump is used in the printing process, additional toner is transferred from the upper sump region.
A toner sensor mechanism is positioned within the lower sump region for continuously monitoring the toner amount. The toner sensor mechanism includes a sensor paddle that rotates within the lower sump and has an angular displacement relative to the amount of toner within the lower sump. When the lower sump region contains an adequate toner amount, the angular displacement is small. When the lower sump has a low toner level, the angular displacement is large resulting in additional toner being supplied to the lower sump.
In one embodiment, the invention includes a toner supply mechanism and toner metering mechanism for supplying toner from the upper sump region to the lower sump region. Both mechanisms are connected via gears to the toner sensor mechanism. The toner supply mechanism includes a dual gear structure having a paddle that extends through the upper sump region for agitating and moving the toner. The metering mechanism includes a metering unit positioned between the lower and upper sump regions for transferring a specific amount of toner. Upon a large angular displacement by the sensor paddle, the gears of the toner supply and metering mechanisms are engaged and transfer a specific amount of toner into the lower sump to allow for continuous printing. This process repeats until all the toner within the upper and lower sumps is depleted.